U.S. Pat. No. 6,533,818 to Weber et al. disclosed an artificial spinal disc including a top and bottom layer fused to the vertebrae and a middle layer formed with polymer having mechanical properties similar to a natural spinal disc.
However, such a prior art has the following drawbacks:    1. The middle layer is made of compressible polymer. Its supporting strength to retain the adjacent vertebrae is doubtful.    2. If the outer support (65) is made of high-strength polymer to increase the supporting strength of the artificial disc, the flexibility of the disc will then become poorer.    3. The polymer layer is fixedly formed between the top and bottom layers between the adjacent vertebrae. It is not suitable to be an implant for minimally invasive surgery.
The present inventor has found the drawbacks of the conventional artificial spinal disc and invented the present artificial intervertebral disc better than the prior art.